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A Level Organic Chemistry: Introduction to nucleophilic addition to aldehydes & ketones

Part 5. The chemistry of ALDEHYDES and KETONES

Doc Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK KS5 A/AS GCE IB advanced level organic chemistry students US K12 grade 11 grade 12 organic chemistry

Part 6.4 Structure and reactivity of aldehydes and ketones - the nucleophilic addition of hydrogen cyanide and the hydrolysis of the nitriles formed and stereochemistry of nucleophilic addition to carbonyl compounds

Sub-index for this page

5.4.1. Structure and reactivity of aldehydes and ketones - nucleophilic attack

5.4.2 The reaction between hydrogen cyanide and aldehydes or ketones to form a nitrile

5.4.3 The mechanism of nucleophilic addition of hydrogen cyanide to aldehydes and ketones

5.4.4. Addition mechanisms - comparing aldehydes/ketones with alkenes

5.4.5 The hydrolysis of the nitrile to a carboxylic acid

5.4.6 The stereochemistry of nucleophilic addition to aldehydes and ketones - implications

INDEX of ALDEHYDES and KETONES revision notes

All Advanced A Level Organic Chemistry Notes

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5.4.1 Structure and reactivity of aldehydes and ketones

Comparison of the structures of aldehydes and ketones carbonyl compounds advanced organic chemistry notes doc brown Structure reminders

Note: An aryl group means a benzene ring C6H5- or a substituted benzene ring e.g. CH3C6H5-.

diagram of pi bond of carbonyl group of aldehydes & ketones nucleophilic attack partially positive carbon atom advanced A level organic chemistry doc brown

Above is a diagram of the sigma and pi bonds of the carbonyl group formed by the overlap of the 2s/2p orbitals of a carbon and oxygen atom.

The pi electron clouds lie above and below the plane of the >C-O sigma bond system, one electron in each pi orbital., formed specifically from the overlap of 2p orbitals

The bond is polar because of the difference in electronegativities of the atoms forming the carbonyl bond, on the Pauling scale carbon is 2.5 and oxygen 3.5.

This produces a shift in the electron clouds towards the more electronegative oxygen atom giving aldehydes and ketones a partially positive carbon atom susceptible to nucleophilic attack by an electron pair donor - a nucleophile e.g. the cyanide ion -:CN (more details further down the page).

The next diagram illustrates the mode of nucleophilic attack on the carbonyl group of aldehydes and ketones.

trigonal planar bonding around carbonyl carbon in aldehydes ketones nucleophilic attack on postive carbon doc brown's advanced A level organic chemistry notes

The geometry of the functional group of aldehydes and ketones

There is a trigonal planar arrangement of bonds around the carbon atom of the carbonyl group (>C=O) because there are three groups of electrons involved.

There are two C-R σ bonds between the carbon and the H/alky/aryl group and the σ plus π of the carbon - oxygen bond of the carbonyl C=O group which spread out in an angular manner of 120o to minimise repulsion.

The planarity of this bond arrangement means that there is a 50:50 chance of the nucleophile attacking on either side of carbon - this has consequences if the product is an optical R/S isomer.

To envisage this, consider the right-hand side of the diagram and imagine the C=O in the plane of the screen and the 'black' bond ▲ pointing directly towards you and the 'grey' bond pointing directly away from you.

This is further discussed with extra mechanism diagrams in section 5.4.6.

 

The reactive nature of the carbonyl group in aldehydes and ketones

Aldehydes and ketones readily undergo nucleophilic attack because of the highly polar carbonyl bond >Cδ+=Oδ caused by the big difference in the electronegativity between carbon (2.5) and oxygen (3.5).

The π electrons

The more electronegative oxygen pulls the bonding electron cloud towards itself producing the highly bond which is the basis for most of the chemical reactions of aldehydes and ketones.

An electron pair donating nucleophile (neutral :Nuc or negative :Nuc-), will therefore attack the 'positive carbon' (Cδ+) to form a C–Nuc bond.

 

The relative reactivity of some aldehydes and ketones

The methyl group (-CH3) has a small electron cloud releasing effect (+ inductive effect →) that partially reduces the effect of the oxygen on creating the delta plus carbon atom.

Methanal has no methyl group, ethanal has one methyl group and propanone has two methyl groups, hence the reactivity trend:

HCHO  >  CH3→CHO > CH3→CO←CH3

From left to right, the delta plus isn't quite as positive!

Generally speaking, aldehydes are more reactive than ketones.

 

Comparison of the mode of reactivity of alkenes compared to aldehydes and ketones.

A comparison of electrophilic addition to alkenes with nucleophilic addition to aldehydes/ketones


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5.4.2 The reaction between hydrogen cyanide and aldehydes or ketones

The product of the nucleophilic addition of hydrogen cyanide is a hydroxynitrile (a cyanohydrin).

The reaction is equivalent to adding H-CN across the C=O bond, to give a N-C-O-H bonding situation.

The addition begins with the initial addition of a cyanide ion (see details of mechanism in  section 5.4.3).

So, the reagent and reaction conditions must be just right.

Hydrogen cyanide is a very weak acid (Ka = 5 x 10-10 mol dm-3), and, on its own at equilibrium, it produces very few cyanide ions:

(i) HCN  +  H2O H3O+  +  :CN-

Therefore a base (alkali) must be present to raise the pH >7.

The presence of a strong base (e.g. hydroxide ion) generates a sufficiently high concentration of cyanide ions to allow the addition of HCN to proceed efficiently.

(ii) HCN  +  OH- H2O  +  :CN-

If the pH is too low, there are insufficient cyanide ions for the reaction to proceed quickly.

hazard The lower the pH, the greater the danger from toxic hydrogen cyanide gas.

In practice, a solution of potassium cyanide (KCN) is used, buffered to about pH ~8.

KCN is the salt of a strong base and a very weak acid, and is naturally alkaline by hydrolysis (the reverse of reaction (ii) above), so providing a higher concentration of cyanide ions than hydrogen cyanide.

To get the right pH, a little dilute sulfuric acid is added to a solution of sodium/potassium cyanide.

Using pure HCN solution, the reaction takes weeks, add a drop of NaOH and it goes in hours, so the base (alkali) has quite a catalytic effect.

 

Examples of nucleophilic addition of hydrogen cyanide to aldehydes and ketones to give hydroxynitriles

(a) aldehydes and ketones nomenclature (c) doc b  +  HCN  ===> 

ethanal  +  hydrogen cyanide  ===>  2-hydroxypropanenitrile

(b) aldehydes and ketones nomenclature (c) doc b  +  HCN  ===>   

propanone  +  hydrogen cyanide  ===>  2-hydroxy-2-methylpropanenitrile

(c) aldehydes and ketones nomenclature (c) doc b  +  HCN  ===>

butanone  +  hydrogen cyanide  ===> 2-hydroxy-2-methylbutanenitrile

 

Need some bigger skeletal formulae equations


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5.4.3 The mechanism of nucleophilic addition of hydrogen cyanide to aldehydes and ketones

The reagent and reaction conditions were discussed above in section 6.4.2.

mechanisms of nucleophilic addition of hydrogen cyanide ion to aldehydes and ketones advanced organic chemistry revision notes doc brown

mechanism 7 – nucleophilic addition of cyanide ion to an aldehyde or ketone

[mechanism 7 above] The >Cδ+=Oδ bond is highly polarised because of the great difference in electronegativity between carbon (2.1) and oxygen (3.5).

Step (1) The nucleophilic electron pair donating cyanide ion attacks the positive carbon of the polarised C=O bond, forming a C–C bond.

The cyanide ion is the nucleophile - donating an electron pair to a partially positive carbon atom.

The lone pair of electrons on the carbon of the cyanide ion, :CN-, forms a C-C bond with the δ+ carbon of the carbonyl group.

Then the electron shift as the π electron bond pair of the original C=O bond moves onto the oxygen to give it a whole negative charge.

Step (2) The negative ion (anion) intermediate formed, RR'C(CN)O, is a strong conjugate base and will abstract a proton from water or hydrogen cyanide to give the hydroxynitrile product and a hydroxide ion.

For step (2) you can also write: RR'C(CN)O  +  HCN  ===>  RR'C(CN)OH  +  CN

but I'm not sure which dominates the proton donation to form the hydroxy-nitrile?

 

diagram mechanism of nucleophilic addition of hydrogen cyanide ion to an aldehyde ethanal nucleophile electron pair donor advanced organic chemistry revision notes doc brown

Mechanism diagram mechanism 77a shows the nucleophilic addition of hydrogen cyanide ion to the aldehyde ethanal.

The initial attacking nucleophile (electron pair donor) on the ethanal molecule is the cyanide ion.

The intermediate anion abstracts a proton from water (or hydrogen cyanide) to form the product 2-hydroxypropanenitrile.

 

diagram mechanism of nucleophilic addition of hydrogen cyanide ion to a ketone propanone nucleophile electron pair donor advanced organic chemistry revision notes doc brown

Mechanism diagram mechanism 77b shows the nucleophilic addition of hydrogen cyanide ion to the ketone propanone.

The initial attacking nucleophile (electron pair donor) on the propanone molecule is the cyanide ion.

The intermediate anion abstracts a proton from water (or hydrogen cyanide) to form the product 2-hydroxy-2-methyl propanenitrile.


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5.4.4 FURTHER COMMENTS

A comparison of the mode of addition alkenes compared to carbonyl compounds

 

Why do alkenes react by electrophilic addition and carbonyl compounds by nucleophilic addition?

Both alkenes (C=C) and carbonyl compounds (C=O) contain π bonds, the π electron cloud is above and below the plane of the C-C or C-O σ bond.

The C=C bond is non-polar, but the C=O bond is, δ+C=Oδ, due to the difference in electronegativity (carbon 2.5 and oxygen 3.5).

Alkene reactivity:

Electron pair donating nucleophiles, especially if negative (e.g. X or OH) will tend to be repelled by the high electron density of the π bond in alkenes.

In alkenes, the electron pair ('rich') donating double bond, is much more likely to react with an electron pair accepting electrophile (Lewis acid) like a positive ion.

Alkenes are also less susceptible to nucleophilic attack because the C=C bond is non-polar.

Aldehyde and ketone reactivity:

Although electron pair donating nucleophiles tend to be repelled by the high electron density of the π bond, in carbonyl compounds, the 'distorted' highly polar >Cδ+=Oδ bond, will be susceptible to attack at the positive carbon (δ+C) by electron pair donating nucleophiles.

The Oδ oxygen atom is not attacked by electrophiles such as HBr or Br2 because the oxygen atom is too electronegative - too reluctant to act as a lone pair donor.

 


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diagram reflux apparatus for hydrolysis of nitrile to a carboxylic acid doc brown's advanced A level organic chemistry revision notes5.4.5 The hydrolysis of the nitrile to a carboxylic acid

The general equation for the hydrolysis of a hydroxy nitrile to a hydroxy-carboxylic acid.

R2C(OH)CN  +  2H2O  ===> R2C(OH)COOH  +  NH3

The nitrile is converted to a carboxylic acid by hydrolysis.

The reaction is slow, so it is speeded up by heating the nitrile with the hydrolysis reagent (e.g. dilute sulfuric acid or aqueous sodium hydroxide) under reflux.

 

(1) Hydrolysis equation with water - very slow, even under reflux.

(a) +  2H2O  ===>    +  NH3

2-hydroxypropanenitrile  +  water  ===>  2-hydroxypropanoic acid  +  ammonia

The products can also be expressed as the ammonium salt

 

(b) +  2H2O  ===>    +  NH3

2-hydroxy-2-methylpropanenitrile  +  water  ===>  2-hydroxy-2-methylpropanoic acid  +  ammonia

The products can also be expressed as the ammonium salt

 

(c) +  2H2O  ===>   +  NH3

2-hydroxy-2-methylbutanenitrile  +  water  ===>  2-hydroxy-2-methylbutanoic acid  +  ammonia

The products can also be expressed as the ammonium salt

 

(2) Hydrolysis equation with acid - good yield under reflux conditions

(a) +  H+  + 2H2O  ===>    +  NH4+

2-hydroxypropanenitrile  +  hydrogen ion ===>  2-hydroxypropanoic acid  +  ammonium ion

Note that this produces the free acid.

 

(b) +  H+  + 2H2O  ===>    +  NH4+

2-hydroxy-2-methylpropanenitrile  +  hydrogen ion ===>  2-hydroxy-2-methylpropanoic acid  +  ammonium ion

Note that this produces the free acid.

 

(c) +  H+ + H2O  ===> +  NH4+

2-hydroxy-2-methylbutanenitrile  +  water  ===>  2-hydroxy-2-methylbutanoic acid  +  ammonia

Note that this produces the free acid.

 

(3) Hydrolysis with alkali - good yield under reflux conditions

(a) +  OH-  +  H2O  ==>    +  NH3

2-hydroxypropanenitrile  +  hydroxide in  ===>  2-hydroxypropanoate ion  +  ammonia

Note that this produces the salt of the acid and the ammonia would be boiled off under reflux conditions.

e.g. from sodium hydroxide you would get sodium 2-hydroxypropanoate.

You add dilute mineral acid (sulfuric or hydrochloric) to free the acid - strong acid displacing a weak one.

 

(b) +  OH-  +  H2O  ==>    +  NH3

2-hydroxy-2-methylpropanenitrile  +  hydroxide in  ===>  2-hydroxy-2-methylpropanoate ion  +  ammonia

Note that this produces the salt of the acid and the ammonia would be boiled off under reflux conditions.

e.g. from sodium hydroxide you would get sodium 2-hydroxy-2-methylpropanoate.

You add dilute mineral acid (sulfuric or hydrochloric) to free the acid - strong acid displacing a weak one.

 

(c) + OH-  + H2O  ==>   +  NH3

2-hydroxy-2-methylbutanenitrile  +  water  ===>  2-hydroxy-2-methylbutanoate ion  +  ammonia

Note that this produces the salt of the acid and the ammonia would be boiled off under reflux conditions.

e.g. from potassium hydroxide you would get potassium 2-hydroxy-2-methylbutanoate

You add dilute mineral acid (sulfuric or hydrochloric) to free the acid - strong acid displacing a weak one.

The acid, here 2-hydroxy-2-methylbutanoic acid, is set free by the addition of a strong mineral acid e.g. dilute sulfuric acid.

CH3CH2C(CH3)(OH)COO-(aq) + H+(aq) ===> CH3CH2C(CH3)(OH)COOH

 

Note on R/S isomers

If the nitrile is derived from aldehydes from ethanal (CH3CHO) onwards and all unsymmetrical ketones (e.g. butanone CH3COCH2CH3), R/S isomers are formed on equal probability basis - a racemic mixture (optically inactive racemate).

In other words, if the resulting nitriles from these carbonyl compounds are hydrolysed, you will also get a racemic (50 : 50)mixture of the mirror image forms of the hydroxynitrile (see section 5.4.6 for more details).

 

Apart from hydrolysis of nitriles to carboxylic acids, they can also be reduced to hydroxylamines.

For details see section Part 5.5 for the details of reduction reactions.


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5.4.6 The stereochemistry of nucleophilic addition to aldehydes and ketones

Implications if the product can exist as R/S stereoisomers

In this nucleophilic addition reaction, at the functional group centre of the reaction (>C=O), you change from an unsaturated trigonal planar situation to a saturated tetrahedral bond network about the carbon atom.

This carbon atom is, in most cases a chiral carbon and the product therefore can exhibit optical R/S isomerism.

However the product is usually a 50:50 mixture of the enantiomers (non–superimposable mirror–image forms) i.e. an optically inactive racemic mixture.

trigonal planar bonding around carbonyl carbon in aldehydes ketones nucleophilic attack on postive carbon doc brown's advanced A level organic chemistry notes

Why is the product an optically inactive racemate even if the product is an asymmetric molecule with a chiral carbon?

The reason can be clearly argued by considering the right-hand of the diagram above.

The nucleophile attacks the carbon of the polarised carbonyl group (R2Cδ+=Oδ) in a trigonal planar bonding situation which changes to a tetrahedral on formation of the C–Nucleophile bond.

Consider the right-hand side of the diagram and imagine the C=O in the plane of the screen and the 'black' bond ▲ pointing directly towards you and the 'grey' bond pointing directly away from you.

Quite simply, there is a 50:50 chance of which side of the carbonyl group the nucleophile attacks and therefore a 50:50 chance of which optical isomer is formed as the configuration about the carbon atom changes.

Apart from explaining the formation of a racemic mixture, you can also argue, in turn, that the lack of optical activity in the product is itself evidence for an initial attack of the nucleophile at the carbon of the carbonyl group and you might reasonably expect a 2nd order rate expression.

rate = k2[aldehyde/ketone][CN]

Though I don't know if the kinetics are actually this simple for what seems to be an initial bimolecular rate determining step mechanism!

 

Picturing this 50:50 in terms of the whole reaction mechanism

diagram stereochemistry of nucleophilic addition of hydrogen cyanide ion to aldehydes ketones carbonyl compounds advanced A level organic chemistry revision notes

Again, imagine the C=O in the plane of the screen and the 'black' bond ▲ pointing directly towards you and the 'grey' bond pointing directly away from you and the NC-C-O bond in the plane of the screen.

If R = R', the molecule has a plane of symmetry and cannot exhibit R/S stereoisomerism - you can't get two non-superimposable mirror image forms of the molecule.

This is the case if the aldehyde is methanal (HCHO) or a symmetrical ketone (e.g. propanone CH3COCH3).

However, for all other aldehydes from ethanal (CH3CHO) onwards and all unsymmetrical ketones (e.g. butanone CH3COCH2CH3), R/S isomers are formed on equal probability basis giving a racemic mixture (optically inactive racemate).

Another consequence of this is if the resulting nitrile is hydrolysed, you will also get a racemic mixture of the hydroxynitriles (see section 5.4.5).


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